PURPOSE: To improve the unstable output due to the deterioration of the S/N at high brightness by designing the load of a photodetector so as to lower the impedance as the DC current increases.

CONSTITUTION: A series circuit comprising a resistor 33 and a diode 50 is provided as a load resistor of the photodetector 7. The impedance of a load diode 50 is high at a low brightness and the entire circuit acts like a conventional circuit. On the other hand, a DC current flows to the photodetector 7 at a high brightness, flows to the resistor 33 and the diode 50 of the load to lower the impedance of the diode 50. The reduction in the impedance act equivalently on both a signal and noise, the signal inputted to the photodetector through a low frequency attenuation capacitor 34 is reduced and the gain of the photodetector is lowered apparently.